CN111386577A

CN111386577A - Method and control system for cooling nuclear reactor core melt

Info

Publication number: CN111386577A
Application number: CN201880043386.0A
Authority: CN
Inventors: A·S·西多罗夫; N·V·西多罗瓦
Original assignee: Science and Innovations JSC; Atomenergoproekt JSC
Current assignee: Science and Innovations JSC; Atomenergoproekt JSC
Priority date: 2018-11-01
Filing date: 2018-12-28
Publication date: 2020-07-07
Anticipated expiration: 2038-12-28
Also published as: JOP20190310A1; BR112019028268A2; US20210358648A1; EP3876243A1; EP3876243A4; JP7255778B2; CN111386577B; KR20200104213A; EA201992737A1; BR112019028268B1; AR116950A1; KR102422554B1; WO2020091623A1; JOP20190310B1; JP2022511137A; US11476010B2; RU2698462C1

Abstract

The present invention relates to a system for ensuring the safe operation of a nuclear power plant in a severe accident, and in particular to a method and a system for the cooling and controlled cooling of the core melt of a nuclear reactor. The invention has the technical result that the safety of the nuclear power station and the cooling efficiency of the molten nuclear reactor core are improved. The task of the claimed invention is to increase the efficiency of cooling the melt in the active zone of a nuclear reactor by safely removing the thermal load on the fusion mirror, ensuring the elimination of steam explosions, thus damaging the accident location zone, the reactor shaft and the containment. The technical result is achieved by modifying the principle of cooling the core melt, which consists in that the conditions of subsequent cooling of the melt after the core of the nuclear reactor vessel has been destroyed by the melt depend on the characteristics of the melt trap body and not of the nuclear reactor. In addition, a temperature sensor and a liquid level sensor are arranged to monitor the cooling process of the nuclear reactor core melt, and technical achievements are achieved.

Description

Method and control system for cooling nuclear reactor core melt

The present invention relates to a system for ensuring the safe operation of a nuclear power plant in a severe accident, and in particular to a method and a system for the cooling and controlled cooling of the core melt of a nuclear reactor.

In a severe accident of a nuclear power plant, the core of a nuclear reactor is damaged and melt enters the lower part of the reactor vessel, thereby damaging the reactor. Destruction of the reactor vessel can lead to undesirable consequences, for example, complicating control of melt distribution and physicochemical behavior. The radioactive decay products in the form of volatile and aerosol diffuse into the containment, compromising its integrity, leaking and causing radioactive contamination of the area. To substantially reduce these negative effects and to eliminate the undesired dose load on the population and environment, modern nuclear power plants are generally equipped with a melt trap under the nuclear reactor, so that, after penetration of the lower part of the reactor vessel, the melt falls into the melt trap, where it is localized and cooled.

Typically, to cool the melt, a coolant (water) is introduced into the melt trap.

In order to control the melt flow after a reactor vessel failure, a temperature sensor, for example a thermocouple designed to monitor the melt flow temperature and its position after the nuclear reactor failure, is installed above the melt trap.

Water should be supplied to the melt inside or outside the reactor vessel to avoid steam explosion in the containment, however, when water is sprayed onto the melt from above, mixing the water with the molten metal, or in the pressure chamber of the melting reactor, pouring the water of the molten metal mixed with the molten oxides into the crust around the molten pool formed in the active zone inside the molten fuel elements, it is not possible to exclude destructive steam explosion, creating nearly ideal conditions for maximum release of maximum energy of the melt due to the dispersion of the melt jet in water, interaction of the dispersion jet and entanglement streams with the equipment surfaces in the reactor pressure chamber or the inner surface of the reactor vessel bottom.

To avoid steam explosion, the melt surface (the melt mirror) is usually not cooled immediately, and cooling is usually performed after receiving information on the state of the melt. Some melt traps are designed in a manner similar to the placement of sacrificial material within the melt trap. After a certain time the composition of the sacrificial material rises above the melt to prevent steam explosion, or in the melt chemically interacting with the sacrificial material the oxide and metal components are inverted, wherein the oxide component of the melt rises and the metal component falls, creating favourable conditions for the supply of water to the surface of the melt (to its oxide component). In some melt trap designs, a special outlet tank is used to divert and distribute the melt, which allows the melt to spread over a large area in a relatively thin layer, allowing spray cooling (blockage) of the melt without the risk of steam explosion. In this case, the water supply is only carried out when the melt is fully diffused in the trap, otherwise, for example, if the melt diffusion mode is violated, the melt accumulates in a limited area, the bottom of the trap may be thermochemically destroyed, or the conditioned cooling water of the steam explosion in the water supply mode melts the top at the trap.

After the reactor vessel has melted, the problem of supplying water to it is no longer considered.

In some nuclear power plant designs, the reactor vessel is filled with water until the reactor vessel melts. Water filling occurs during the active zone destruction phase, with melt flowing from the active zone to the bottom of the reactor vessel where it accumulates until the bottom is destroyed. This procedure is very dangerous. The reason for this is the steam explosion, when water is mixed with the liquid metal and with the liquid oxide, both are present in the melt in pure form, occurring only in a certain amount of the mixture of liquid metals.

On the other hand, there is uncertainty about the cooling water supply to the reactor vessel due to the lack of information about the location of the melt within the reactor vessel. Therefore, the water supply to the reactor vessel cannot ensure its safety.

The reactor control system measures the neutron flux outside the reactor vessel without damaging the core, and monitors for changes in core reactivity, power changes and other parameter changes during normal operation, conditions of violation of normal operation, conditions during design, and conditions of out-of-design-benchmark accidents. The system is not specially designed for monitoring serious accidents of the nuclear power plant, and factors in the serious accidents of the nuclear power plant, such as the change of the arrangement of elements of a reactor core and solid fragments thereof, the change of the element arrangement of internal devices, the change of the position and the volume of a reactor core melt in a reactor vessel, the change of chemical components and phase states thereof, including the formation and separation of sludge (two-phase solid-liquid state) and the change of a heat engine state thereof, cause the remarkable distortion and change of physical parameter data of inner and outer reactors of the reactor vessel.

A known method [1] of cooling a nuclear reactor active zone melt comprises determining, after a nuclear reactor containment active zone melt breach, the position of core melt fragments in a reactor vessel and determining the state of core melt from information received from a temperature sensor installed in the nuclear reactor, supplying a coolant to the reactor core, increasing or decreasing, in view of the received information, the volume of coolant supplied to the reactor core after the reactor vessel melt, supplying the reactor core with coolant, taking into account the actual position and condition of the nuclear reactor core molten elements.

A known system for implementing the method [1] comprises a first set of temperature sensors installed above the core of the nuclear reactor, a second set of temperature sensors installed outside the nuclear reactor vessel, a third set of temperature sensors installed at the bottom of the nuclear reactor vessel, a fourth set of temperature sensors installed in the area between the melt trap and the bottom of the nuclear reactor vessel, connected to a control device.

One of the disadvantages of this method, and the system implemented for it, is that the active oxidation of zirconium starts with evolution of hydrogen at temperatures above 1100 ℃. The temperature in this process is rapidly increased from 1200 ℃ to 1800-2200 ℃ or higher. This results in destruction of the temperature sensor installed in the reactor vessel and, essentially, only the moment at which the destruction of the core begins is allowed and the region in which the destruction process is faster is approximately located on the basis of the data of the temperature increase and the data of the sensor failure. A temperature sensor mounted in the reactor vessel above the core will indicate the temperature of the steam gas medium (the temperature of the steam/hydrogen mixture) over a period of time, which is distorted by the cyclic processes within the core. Due to the particularities of the core design, these sensors can exhibit a fairly acceptable temperature for a fairly long time, which enables the steam-gas mixture to circulate through several almost independent peripheral channels, which results in a significant underestimation of the average temperature of the gas-steam mixture above the core compared to a similar temperature in her channel.

Another drawback of this method, and the system designed to implement it, is that the temperature sensors installed on the external surface of the reactor vessel do not allow to determine the state of the active zone, due to the thermal inertia of the reactor vessel and the distortion of the temperature field caused by the processes inside the reactor, related to combined cycle convection, core melting, re-radiation, and other thermo-chemical and thermo-hydraulic processes. Thus, some changes can be detected by temperature sensors mounted on the outer surface of the reactor vessel, but this information is clearly insufficient to determine the state of the active zone, in particular the state of the melt, without additional data relating to the parameters of the medium in the primary and containment zones.

Thus, external control of the state of the active zone in the reactor vessel is not an independent control and cannot be effected alone.

Thus, due to the lack of reliable information about the state and location of the melt within the reactor vessel, cooling the melt by supplying a coolant (water) to the reactor vessel becomes impossible, as this would not only lead to a steam explosion, but also to destruction of the containment, which would result in the release of radioactive fission products beyond the boundaries of the nuclear power plant site.

The technical result of the invention is to improve the safety of the nuclear power station. Efficiency of cooling of nuclear reactor core melt.

The task of the claimed invention is to increase the efficiency of cooling the melt in the active zone of a nuclear reactor by safely removing the thermal load on the fusion mirror, ensuring the elimination of steam explosions, thus damaging the accident location zone, the reactor shaft and the containment.

This problem is solved by a method for cooling the melt in the active area of a nuclear reactor, comprising determining, according to the present invention, after the melt ruptures the core of the nuclear reactor core, the position of the core melt fragments based on information received from a temperature sensor, and determining the penetration state of the core, the supply of coolant, increasing or decreasing the supply of coolant, determining the degree of destruction of the nuclear reactor vessel and the start time of entry of the melt flowing out of the nuclear reactor vessel into the trap alloy after the core melt ruptures the core, then feeding coolant into the melt catcher body with a predetermined time delay from the manhole and the guard tube block of the nuclear reactor inner shell device, then determining the condition of formation of slag caps above the surface of the melt mirrors, determining the start time of the melt surface skinning, determining the end time of aerosol release, determining the completion time of vapor sorption and the time of hydrogen generation, the settling time of the melt cooling processes and the time for these processes to enter the quasi-static mode are determined, then the coolant supply is adjusted taking into account the thermophysical parameters of the medium in the sealed vessel volume, and then the coolant supply is adjusted taking into account the minimum and maximum water levels in the reactor shaft.

A control system for cooling the melt of the core of a nuclear reactor, comprising temperature sensors connected to the control device according to the invention, and furthermore comprising a level gauge installed under the farm of the control console, the level gauge being installed along the melt catcher body of its external water-cooled zone, the temperature sensors being divided into a first group, a second group and a third group, the first group of temperature sensors being installed above the melt mirror in the melt catcher body, the working body thereof being directed towards the melt mirror, the second group of temperature sensors being installed between the melt catcher body and the truss control console; the third group of temperature sensors are arranged below the guide plate, all the temperature sensors and the liquid level meter are combined in the two channels, a sealed limit switch is arranged on a working body of each temperature sensor, and a leakage protective cap covers the switch.

A significant feature and fundamental difference between the claimed method and prototype is that the status of the melt trap is monitored when the core is damaged, rather than the in-reactor space, since the reactor vessel as a part (and the entire first loop as a whole) is a power generation system, in relation to the containment (strength and density during normal operation), the melt trap is an open energy generation system built into the containment that allows in-containment monitoring and control procedures to ensure effective impact on the melt trap.

A similar procedure cannot affect the pressure chamber during the internal reactor because of any damage to the primary circuit up to the maximum design basis (the primary circulation tube is broken at full section), the reactor chamber remains a closed system with respect to the pressure jacket, one of the features is that there is a certain residual pressure in the reactor vessel with respect to the pressure in the containment vessel, which does not allow for effective indirect control of the process in the reactor vessel by changing the process parameters in the containment vessel.

A significant feature of the claimed system is that the temperature sensor and the level sensor are divided into two channels, mounted on the melt trap body, rather than on the body of the nuclear reactor, which allow control and regulation during cooling of the nuclear reactor core melt.

Another significant feature of the claimed system is that the temperature sensors are mounted at three different levels with respect to the fused mirror, which ensures that each channel receives equivalent characteristic information.

Another notable feature of the claimed system is that all temperature sensors located in the melt trap body or in the direct and indirect melt action zones have protective end caps to provide thermo-chemical and gas dynamic protection for their working bodies.

Another notable feature of the claimed system is that all limit switches of the temperature sensor are mounted in a leaktight cover that provides thermo-mechanical protection against splashing or small amounts of molten core, liquid concrete and its component spray, small flying objects and aerosols.

FIG. 1 shows a flow chart of a method of cooling nuclear reactor core melt.

Fig. 2 shows a control system for cooling the melt in the core of a nuclear reactor.

Figure 3 shows a protection-sealed trailer and protective cap for a temperature control sensor.

The working principle of the invention is as follows.

The process of cooling the melt of the nuclear reactor core comprises several main phases:

determining (1) the extent of damage to the nuclear reactor vessel and the start time for the flow of melt out of the nuclear reactor vessel into the melt trap;

supplying coolant from the nuclear reactor internals and the manholes of the protection tube blocks with a predetermined time delay into (2) the melt trap body;

determination of the Condition for forming a slag Cap over the surface of the melting mirror (3)

Measurement of the time to start skinning on the melt surface (4);

determination of the end time of aerosol release (5);

determination of the time to completion of the steam adsorption and hydrogen formation (6);

determination of the stabilization time of the melt cooling process (7);

determining (8) the times at which these processes enter the quasi-stationary mode;

increasing or decreasing (9) the coolant supply, taking into account the thermophysical parameters of the medium in the containment volume;

the coolant supply is increased or decreased (10) taking into account the minimum and maximum water levels in the reactor shaft.

The essence of this process is as follows. After the nuclear reactor core is melted, the core melt begins to flow onto the flow guide plate and down into the melt catcher. Before this process, there are two events that determine the subsequent control measures: the first event is the flow of primary coolant and cooling water from the primary and non-primary systems to the filter chamber (in the pool) associated with the reactor shaft in which the melt trap is installed, and the second method is to first heat the air inside the melt trap and then heat the gaseous medium. For example, coolant outflow may first result from a first event due to a primary circuit pipe rupture, subsequent failure and normal operation of the active safety system, or due to a primary circuit pipe rupture and a complete power outage at the nuclear power plant. The water in the passive safety system is then cooled to a pressurized volume. This water forms a combined water level in the reactor shaft around the filter chamber and the melt trap body, which is diagnosed by a set of level gauges mounted under the control console along the melt trap body in its outer water cooled zone. The water enters the filtration chamber and enters the reactor shaft, connected to it by a section at the bottom. The level gauge diagnoses the presence of water around the melt trap body, while the temperature sensors at the three levels indicate that the temperature in the melt trap during this period does not exceed 400 ℃, which is related to the absence of core melt in the reactor vessel. The gradually cooled water boils out of the reactor vessel, and the core heats up, collapses, melts, and flows to the bottom of the reactor vessel. But even in this case the temperature sensors located below the bottom of the reactor vessel show temperatures below 400 c because they are protected by the guide plates and truss control console. All gas convection from the heating box is much higher than the temperature sensor is located in the relatively cold heated region, which remains at a constant temperature due to the quasi-steady state temperature of the water in the filtration chamber.

When the reactor pressure vessel is broken, the following process occurs: in the first phase, the pressure changes in the melt and the liquid metal with a certain amount of liquid oxide enter the VLR packing, while the temperature sensors of the first, second and third group are either heated above 400 ℃ or are destroyed by the final melt, i.e. are all in a state of failure. According to two of these indications: overheating of the temperature sensor or failure (destruction) of the temperature sensor determines (1) the onset of core melt entry into the melt trap from the nuclear reactor vessel. In fact, according to these two characteristics, the degree of damage of the reactor vessel and thus the staged flow of the melt from the reactor vessel into the melt trap are also determined, namely: ) The liquid metal first flows out, then the liquid oxide flows out after a period of time, which indicates that a side penetration of the reactor vessel exists; b) a volume of the entire melt flowed out of the reactor vessel at the same time, indicating a disruption of the bottom of the reactor vessel. Both of these conditions are determined by the readings obtained from the temperature sensors of the first, second and third groups, namely:

a) if after damage to the reactor vessel the temperature sensors of the first, second and third group show a temperature exceeding 400 ℃ after which the temperature continues to rise slowly, after a few hours, for example after 2-3 hours, the temperature rises rapidly, which means a two-stage process of side penetration of the reactor vessel and melt entry (first liquid metal flowing out, then liquid oxide flowing out), whereby the water from the wells of the internals and protection tube blocks is provided with a designed (preset) delay, for example a delay of three to four hours, providing confirmation of melt-dissolved packing, and reversal of the inclusion composition (oxides above and metal at the bottom) has occurred;

b) if the temperature sensors of the first, second and third groups show a temperature exceeding 400 c after reactor vessel failure, immediately after which the temperature starts to rise uniformly or gradually rapidly, which means that there is a phase process of melt arrival (melting of the metal with the molten oxide), and therefore water from the wells of the inner shell means and the protective tube block is provided earlier in time, roughly in the range of 30 minutes to 1 hour from the moment of melt arrival, because the filler dissolves fast enough and the reversion of the metal and oxide occurs rapidly within 30 minutes.

Accordingly, based on the readings of the first, second, and third sets of temperature sensors, a timer for delaying the supply of water from the inspection chamber and the protective tube blocks of the nuclear reactor internal shell apparatus to the molten surface is turned on, and then (2) coolant is supplied within the molten well. The delay time may be set from 30 minutes to 4 hours. The delay time is determined taking into account the duration of the outflow of the oxide fraction of the melt from the reactor vessel (in the case of a two-stage jet discharge in a hole in the damaged side of the reactor vessel). Furthermore, in order to set the delay time, the volume of sacrificial steel and the volume of sacrificial oxide in the filler are generally taken into account, which is necessary to transfer the high temperature and chemically aggressive core melt to a steady state. This allows the melt to be cooled without damaging the melt trap body and without additional damage from thermal radiation from the truss console and the guide plate.

One important point for ensuring the passive safety of the melt is the inversion of its oxide and metal components, which occurs when the filler melts and dissolves in the bore liquid due to the reduced density of the filler relative to the oxide fraction of the metal melt. As a result of the inversion, part of the melt containing uranium oxide floats and the metal part of the melt falls. Inversion of melt composition can solve two problems:

1. in order to ensure a uniform heat flow from the core melt through the melt trap body to the water, the maximum heat flow distribution of the initial heat flow distribution in the region of the location of the steel melt above the molten oxide is smoothed and aligned with the height of the trap body after the uranium-containing oxide has been deposited on the steel melt. The alignment of the heat flux after inversion is ensured mainly due to the difference of the thermal physical properties of the oxide melt and the metal melt.

2. The melting mirror is cooled directly with water to suppress the activity and thermal radiation of the molten aerosol from the melting mirror to the above located catcher body equipment, to the truss console and guides that secure the bottom of the reactor body with the core and internals fragments.

During the interaction between the nugget and the filler, a slag cap of light filler oxide is formed over the molten mirror. The slag cap reduces the interaction of the open liquid metal surface with water vapor, producing hydrogen gas. Further, with the above apparatus, the slag cover reduces heat radiation from the side of the melting mirror. When the slag cap cools, a crust forms on it. Determining (3) formation of slag caps and encrustations using a first and a second set of temperature sensors that exhibit periodic temperature fluctuations, namely: if the crust size increases, a slight temperature drop will result, and if the crust breaks, the temperature rises sharply due to the release of gas and aerosol. Subsequent water supply to the melt surface reduces its surface temperature. An operational temperature sensor located at each of the three levels on the melt trap displays temperature drop data after water is supplied to the melt surface from the inner shell means and the inspection chamber of the protective tube block. Based on the readings of the temperature sensor located in the three layers (to lower the temperature), they determine (4) when the aerosol outlet stops, i.e.: the water entering the melting surface prevents the exit of the aerosol and the thermal radiation, also rapidly cools the equipment located above and stabilizes its mechanical characteristics, which therefore leads to a temperature reduction.

The completion time of the vapor adsorption and hydrogen formation is determined from the readings of the temperature sensors at three levels (5). These readings coincide with the aerosol release stopping and the melting mirror starting to water cool.

Then, based on the proof of all operable temperature sensors, determining (6) the stabilization times of the melt cooling processes and determining (7) the times at which these processes transition to the quasi-steady-state mode, i.e.: if the temperature sensor shows that the temperature remains constant during the gradual cooling of the melt and subsequently decreases, this indicates that there is a steady cooling process of the melt, in which the average temperature of the melt decreases with decreasing release of residual energy, which solidifies and passes from the liquid phase to the solid phase.

The remaining melt at the bottom of the reactor vessel and at the bottom itself is also gradually cooled. The stabilization and cooling are diagnosed by a temperature sensor, which is positioned on the third layer and displays the temperature of the steam medium under the guide plate. These readings are affected by the re-radiated heat flux from the thermal surfaces of the internal components of the truss console and the lower plane of the guide plate, the lower the re-radiation, the lower the temperature of the third set of sensors, the cooler the truss console and guide plate surfaces, the lower the temperature of the guide plate and the core residue on the guide plate. Increasing or decreasing the water supply of the melt catcher after the water supply of the inspection well of the inner housing arrangement and the protection tube unit is completed (8) based on the readings of the third set of temperature sensors, i.e.: if the temperature starts to rise after the water supply to the inspection chamber and the protective tube block of the inner housing device is stopped, the volume of water supplied to the melt trap body increases, and if the temperature does not rise, the volume of water supplied to the melt trap body decreases or stops completely.

They increase or decrease the water supply (9) according to the level meter (13) reading, taking into account the minimum and maximum water levels in the reactor shaft. The water level is related to the water level at which the melt catcher body flange and the truss console base are located, namely: the water supply is increased if the water level is below the water level of the housing flange, and the water supply is decreased or water flow into the melt trap body is completely stopped if the water level is at the water level of the truss console base.

As shown in figures 2 and 3, the control system for cooling the melt in the active zone of a nuclear reactor comprises a first, a second and a third group of temperature sensors (10, 11, 12) and a group of level sensors (13) combined in two channels (14) and connected to a control device (15), on the working body (16) of each temperature sensor a sealed limit switch (17) is mounted and closed by a leakage protection cap (18), the first group of temperature sensors (10) is mounted above the melt mirror (19) in the body (20) of the melt trap (21) and the working body (16) thereof is guided towards the melt mirror (19), the second group of temperature sensors (11) is mounted between the body (20) of the melt trap (21) and a truss console (22), the third group of temperature sensors (12) is mounted under a guide plate (23), the group of level meters (13) is mounted under the truss console (22), is mounted along the body (20) of a melt collector (21) in its outer water-cooled zone.

At the moment of failure of the reactor vessel (24), the melt (25) of the active zone starts to flow into the inner shell (20) of the melt trap (21) under the effect of hydrostatic and excess pressure and comes into contact with the packing (26).

The filler (26) provides a volumetric dispersion of the globular melt (25) within the inclusions (21) and serves to additionally oxidize the dermis and its dilution to reduce the volume of energy and increase the heat transfer surface of the energy producing dermis with the outer layer of the crater (21) while also helping to create conditions for the fuel containing portion of the dermis to rise above the steel layer. The filler (26) may consist of steel and oxides containing oxides of iron, aluminum, zirconium, the channels of which redistribute the dermis not only in the cylindrical part but also in the bottom conical volume.

The steel and oxide components are completed in a cylindrical barrel. Often, the packing includes at least a first cylinder mounted at the bottom of the trap body, a second cylinder located above the first cylinder, and a third cylinder mounted above the second cylinder. The third cartridge may in turn consist of several cassette cartridges mounted on top of each other.

In practice, three sets of temperature sensors (10, 11, 12) are mounted on three levels, whereas the first set of temperature sensors (10) is mounted within the body (20) of the melt trap (21), and the second and third sets of temperature sensors (11, 12) are arranged above the body (20) of the melt trap (21).

A first set of temperature sensors (10) is located at a closest distance from the melt mirror (19) and slag cap (27) to provide temperature control. Above these temperature sensors is thermal protection (28) which provides protection against the effects of flowing melt and flying objects. The working body (16) of the temperature sensors (10) faces the melt (25). The first set of temperature sensors (10) stops after the melt mirror (19) is formed in the body (20) of the trapway (21), because at this point the heat radiation from the sides of the melt mirror (19) begins to melt the heat shield from below (28). The temperature rise of the melting mirror to the melting temperature of the thermal protection indicates that at this point in time a reaction between the melt and the filling takes place, and in addition, the heat transfer through the body of the trap to the coolant and the heat transfer also take place in quasi-steady-state mode, the melt of the trap (21), the truss console (22) and the guide plate (23) of the device of the transition radiation upstream element.

A second set of temperature sensors (11) mounted between the melt catcher body (20) and the truss console (22) also provide temperature control. These temperature sensors (11) are located in an area not protected by the heat shield and the heat shield. The second set of temperature sensors (11) operates according to the nature of the core melt entering the melt catcher from the reactor vessel: for example, the molten steel may be rapidly non-axisymmetrically fed in for about 30 to 60 seconds and have a mass of about 60 to 100 tons, or the molten liquid oxides may be slowly non-axisymmetrically fed in a certain amount of the molten steel mixture, for example, for 2 to 3 to three hours, weighing about 90 to 130 tons, and some of the temperature sensors of the second group are partially melted (failed). However, some of the temperature sensors (11) continue to operate even after the axial-symmetric outflow of the melt from the reactor vessel is completed, which is the most difficult for the operability of the second set of sensors. From the readings of these temperature sensors, the time at which the bottom of the reactor vessel starts to break, i.e. in fact, the time at which the melt starts to flow out, and the subsequent state of the vaporous gaseous medium above the melt surface or its envelope, are determined, among the most important parameters. Based on these data, a timer is activated to automatically provide the cooling fluid with a predetermined delay. Coolant is supplied into the drain valve body from the inspection shaft and the protection pipe block of the internals to cool the slag cap and the core melting mirror therebelow.

A third set of temperature sensors (12) mounted at a closest distance from the reactor vessel (20) also provides temperature control. These temperature sensors (12) are installed in a protected cooling zone located below the guide plate (23) and remain operational throughout the cooling of the active zone melt in the trapway body. From the readings of these sensors (12), the time at which the bottom of the reactor vessel starts to break, i.e. the time at which the melt starts to flow out and the subsequent state of the vaporous gaseous medium above the melt surface, are determined, which are one of the most important parameters. Based on this data, a timer is activated to automatically provide the cooling fluid with a predetermined delay. Coolant is supplied from the inspection shaft and the protection pipe unit of the inner housing device into the trap body to cool the core melt. Further, based on the readings of these sensors, the formation of a slag cap above the surface of the melting mirror is recorded, the start time of the formation of a crust on the surface of the melting mirror is determined, and information about the termination of the aerosol release and the completion of the vapor adsorption and hydrogen formation processes is received.

A set of level gauges (13) is mounted at least at two external levels outside the housing (20) of a melt collector (21) located in the cooling zone of the housing (20) for controlling the coolant level in the reactor shaft. The set of level gauges (13) is located in a protected cooling area under the truss console (22). From the readings of the level gauge (13) the level of the bay with water on the reactor shaft is determined, i.e. the design function of the trap cooling system is determined from these readings, or the operation of the system is adjusted.

The method for cooling the melt in the active area of the nuclear reactor and the application of the control system for cooling the melt in the active area of the nuclear reactor make it possible to improve the efficiency of cooling the melt in the active area of the nuclear reactor by eliminating the thermal load on the melt mirror, and in turn completely eliminate the possibility of steam explosion in the heat removal process of the melt, thereby improving the safety of the nuclear power station.

Reference documents:

1. china patent No. CN106651217, IPC G21D3/06, priority date 1/6 of 2017.

Claims

1. A method of cooling a melt in a nuclear reactor core, comprising determining the position of molten core fragments after the core melt has ruptured the core of a nuclear reactor vessel, determining the core penetration state based on information received from a temperature sensor, supplying and adjusting the coolant supply volume to the reactor for melting, characterized in that after a melt rupture in an active zone of the nuclear reactor vessel, the degree of rupture of the nuclear reactor vessel and the melt flow time of the nuclear reactor in the outer shell of the melt trap, then feeding coolant into the melt trap with a predetermined time delay from a manhole and a containment block of an inner shell device of the nuclear reactor, then determining the condition of forming a slag cap above the surface of the melt mirror, determining the start time of skinning on the melt surface, determining the end time of aerosol release, determining the completion time of steam adsorption and the time of hydrogen generation, the settling time of the melt cooling processes and the time for these processes to enter the quasi-static mode are determined, then the coolant supply is adjusted taking into account the thermophysical parameters of the medium in the sealed vessel volume, and then the coolant supply is adjusted taking into account the minimum and maximum water levels in the reactor shaft.

2. A control system for cooling the melt in the active zone of a nuclear reactor, comprising temperature sensors connected to control equipment, characterized in that it also comprises level sensors installed under the truss control console, along the main body of the melt trap in its external water-cooled zone, the temperature sensors being divided into a first group, a second group and a third group, the first group of temperature sensors being installed above the melt mirror in the melt trap body with its working body facing the melt mirror, the second group of temperature sensors being installed between the melt trap body and the truss control console, the third group of temperature sensors being installed under the guide plate, all the temperature sensors and level sensors being combined in two channels, sealing end caps being installed on the working body of each temperature sensor protecting the leakage cover.